The present invention relates to a liquid mixing collector for capturing and mixing liquid descending from an overlying zone in a mass transfer or heat exchange column. The present invention also relates to a mass transfer or heat exchange column comprising said collector, use of said collector in a mass transfer or heat exchange column, and a method of collecting and mixing descending liquid in a mass transfer or heat exchange column.
Mass transfer columns, including heat exchange columns, typically include a shell and a plurality of zones within the shell in which packing and/or trays are used to facilitate mass or heat transfer between fluid streams flowing within the column. The fluid streams are normally one or more downwardly flowing liquid streams and one or more ascending vapor streams, although other combinations of fluid streams may be possible. Liquid exiting the bottom of a zone may have different concentrations and compositions at different locations across the horizontal cross section of the zone. In order to reduce these concentrational and compositional maldistributions, the liquid is often collected and mixed before it is then distributed to an underlying zone. To avoid detrimental maldistribution effects, the packing may be split in a number of beds, and the descending liquid may then be collected and redistributed over the cross section of the column in each portion between two beds. Separate collecting, mixing and distributing components are often used to carry out the desired collection, mixing and distribution of the liquid as it descends from one zone to another.
For example, the redistribution is classically achieved by means of a mixing drum. The use of separate components, however, can be undesirable because the vertical spacing occupied by each of these components reduces the available area within the column for other processing of the fluid streams such as the mass or heat transfer and may require that a taller and thus more expensive column be utilized to provide the spacing needed to effect the desired processing operations.
In particular, in order to achieve the best efficiency, beds of structured packing should not exceed a certain height. Increasing the bed height or increasing number of theoretical stages makes the bed prone to effects of maldistribution. As discussed earlier, maldistribution has a negative effect on the separation efficiency and the achievable purity of the distillate stream that is drawn at the top of the column. This aspect has a very high importance in the distillation of mixtures with very close boiling components because such a task can only be achieved with a large number of stages. Examples of such distillations include the separation of styrene monomer from ethylbenzene.
Chimney trays are a well known type of liquid collector that is placed below packing beds in columns of the type described above. They are used to collect the liquid that drains from the packing bed and to further convey it to the next packing bed. Such chimney trays consist of a flat horizontal portion to accommodate liquid. The liquid is collected in a downcomer which guides it into a pipe. The horizontal portion includes openings in the shape of chimneys through which vapor is allowed to rise. The chimneys feature a “cap” or “roof” which collects descending liquid and leads the liquid onto the horizontal portion of the tray.
There exists an alternative type of liquid collector, namely the vane collector. The vane collector has a much simpler construction than the chimney tray, but it is only useful for low liquid loadings. For applications with high loading (typically above 25 to 30 m3/m2h), the chimney tray is preferred due to its more flexible set-up. For example, increasing the height of the chimneys allows for a higher liquid level on the tray, and the flexible spacing between chimneys allows this spacing to be increased to give a larger area that may then be occupied by liquid on the tray.
Thus chimney trays can be flexibly engineered to deal with various types of liquid and vapor loads. However, this flexibility has the disadvantage of increasing the complexity of the construction. For example, trays must be tight such that no liquid penetrates through seams between the chimneys and the horizontal portion of the tray. As a consequence, chimneys are built as welded parts which are then also welded onto the horizontal portion of the chimney trays. Thus, chimney trays are a rather expensive liquid collector design to construct.
Both chimney- and vane-type liquid collectors may be used with mixing drums, as discussed earlier; however, the use of these separate liquid collecting and mixing components requires then a complex construction that disadvantageously takes a lot of additional space between the beds and raises the height of the column. Alternatively instead of using a separate mixing drum, a chimney tray combined with a mixing box installed on top of the chimney tray is known from EP 0 879626 A2. However, the use of a mixing box nonetheless still requires considerable vertical spacing in the interbed section, the disadvantages of which have been discussed earlier. It is noted that a special vane collector in which both liquid collecting and mixing functions are combined is known from U.S. Pat. No. 7,114,709 B2; however, this special vane collector still suffers from the disadvantages of vane collectors, for example, in applications having high loadings.
In conclusion, it would be desirable to have a chimney tray having combined liquid collection and mixing functions, especially one that is less expensive and requires less welding to construct, and particularly one for use in applications having high loadings.